Desperately need help please?

Theres this girl i like, shes 16, year below me. I got to know her and went out with her once but got dumped the next day. We didnt speak for a while but a few weeks ago she came back into my life. The reason for this is that my mum is best friends with her mum and my sister is friends with this girl's sister. The girls sister is ill, so we always travel up to town to meet them, and we always hang about together and have a laugh, the thing is i still desperately like her, which is frustrating because i cant keep away as our families are so close, its like a reminder every time that i cant be with her, bloody frustrating. If any one can help it would be much appreciated.